In a tandem color image forming apparatus, such as a color electrophotographic printer, four color image forming units are arranged in a feeding direction of a recording medium. The four color image forming units respectively form color images of black (K), yellow (Y), magenta (M), and cyan (C) on a recording medium by using toners of each color. Therefore, in such a color image forming apparatus that employs the electrophotographic method, since a toner image is formed at a shifted position instead of its proper position because of mechanism size and drive system error, image unevenness and distortion as well as a color shift in which four color toners are not positioned properly, causing the created images to be unclear. As a means to correct the color shift that occurs in the color image forming apparatus, forming a detection pattern (i.e., a toner mark), as a position shift (or positional gap) detection mark, in a predetermined interval on a carrying belt that feeds a recording medium, reading the formed detection pattern by using a detector, calculating an amount of position shift based on the reading result, and correcting an image forming position depending on the calculated amount of position shift, are performed.
When the amount of position shift is calculated by sensing the detection pattern on the carrying belt, various kinds of shifts occur depending on a position of a recording medium in the feeding direction, and an error occurs in detection of the amount of position shift. For example, when an eccentricity and rotation unevenness (i.e. change in rotation speed) occur on a photosensitive drum in the image forming unit and a driving roller of the carrying belt, the amount of position shift changes depending on each position of the recording medium in the feeding direction. In particular, when the eccentricity and the rotation unevenness occur on the photosensitive drum, the amount of position shift changes with a period of one circumference of the photosensitive drum, as a basic frequency, in each color. Therefore, the amount of shift changes depending on the sensing position.
To solve the trouble above, Japanese laid-open patent application 2006-078691 judges a relative phase difference as an optimal value of the change. The relative phase difference minimizes the change of the print position shift (PPS) value periodically caused by the relative eccentricity of the photosensitive drum and a driven gear, namely, more specifically, by changing the engaged positional relationship multiple times, from the position shift detection value of each variation in every multiple changes by varying the position of the detection pattern, as the position shift detection mark, on the carrying belt according to the relative phase difference of each photosensitive drum etc. so as to detect the PPS value at a number of positions.